I believe "dead key" refers to those keys that you press first to change what happens when you press the next key ... what I mean is for example to get á¾¶ you first press '[' and then 'a', where '[' is the dead key that puts a circumflex accent on the following letter (if possible). With the Greek Polytonic keyboard, though, there are a lot of dead keys, because of how many different combinations of accents, breathings, and so on are possible.